1. Field of the Invention
The invention relates to a method and apparatus of inspecting welds and, more particularly, inspecting welds in an upper core shroud of a reactor vessel of nuclear power plant.
2. Description of the Prior Art
In a nuclear power plant, the nuclear reaction occurs inside of a reactor containment vessel which further has a reactor vessel therein. Inside of the reactor vessel is located a core shroud in which a nuclear reaction occurs. The inside of the core shroud is subjected to wide variations in temperature and pressure. As a result of the wide variations in temperature and pressure, metal fatigue could occur in the core shroud. To ensure that does not occur, or monitor potential problems if they do occur, there are requirements by the Nuclear Regulatory Commission that the core shroud be periodically inspected, especially any welds in the core shroud.
In the past, various types of inspection devices for inspecting the core shroud have been developed such as is shown in U.S. Pat. No. 5,586,155 to Erbes et al. However, the invention as shown in the Erbes patent has some practical problems. The assembly mounts on the steam dam and is propelled around the steam dam by conical tractor drive wheels. Because of the slippage of the tractor drive wheels, the operator at the top of the reactor containment vessel cannot tell exactly where the sensors are located within the core shroud.
The inspection of the core shroud occurs when the particular reactor of a nuclear power plant is shut down. While that reactor is shut down, the top of the reactor containment vessel is opened, the top of the reactor vessel is opened and the top of the core shroud is opened. Due to the nuclear radiation, the person performing the inspection has to remain at the top of the opened reactor containment vessel. The inspection device must be operated entirely from the top of the reactor containment vessel. Typically at the time the inspection is being made of the core shroud and the welds therein, numerous other activities are occurring in the shut-down unit of the nuclear power plant. Therefore, numerous people performing other functions will be at the open top of the reactor containment vessel. Hence, space at the top of the reactor container vessel is limited.
One of the problems that existed for prior inspection methods of a core shroud is that they required a ring to be mounted all the way around the top of the core shroud, typically on the steam dam. This meant a lot of room had to be taken at the top of the reactor containment vessel during the period of shut down, which is when other people are needing space to perform their functions. Also, the shroud at the steam dam was not perfectly circular and many times the rings would not fit on the steam dam.
In addition to the Erbes patent described herein above, other patents have been published and/or issued on various tools that can be used to inspect core shrouds from the top of the containment vessel. Such patents or patent applications include Johnson (U.S. Pat. No. 6,332,011), Ortega (U.S. Patent Publication No. US 2008/0165911), Morris (U.S. Patent Publication No. US 2007/0125190) and Morris (U.S. Patent Publication No. US 2008/0205575). Each of these patents show various types of ways of inspecting core shrouds located within a reactor vessel of a reactor containment vessel.